The invention relates to a multi-section laser diode that can be switched between different wavelengths, more especially to a laser system comprising a control circuit that enables a multi-section laser diode to be switched rapidly between different wavelengths.
A typical multi-section diode laser is a three-section tunable distributed Bragg reflector (DBR) laser. Other types of multi-section diode lasers are the sampled grating DBR (SG-DBR) and the superstructure sampled DBR (SSG-DBR) which both have four sections. A further multi-section diode laser is the grating-assisted coupler with rear sampled or superstructure grating reflector (GCSR), which also has four sections.
FIG. 1 is a basic schematic drawing of a SG-DBR 10. The laser comprises back and front reflector sections 2 and 8 with an intervening gain or active section 6 and phase section 4. An antireflection coating 9 is usually provided on the front and/or rear facet of the chip to avoid facet modes. The back and front reflectors take the form of sampled Bragg gratings 3 and 5. The pitch of the gratings of the back and front reflectors is slightly different, to provide a Vernier tuning effect through varying the current supplied to these sections.
The phase section, for example by refractive index changes induced by varying the carrier density in this section. A more detailed description of the SG-DBR and other tunable multi-section diode lasers can be found in PCT patent publication number WO03/023916 entitled “Multi-section Diode Lasers”.
Multi-section diode lasers may be useful in wavelength division multiplexed (WDM) systems. Example applications are as transmitter sources, as wavelength converters in optical cross connects (OXCs) and for reference sources in heterodyne receivers. Typically, WDM systems have channel spacings conforming to the International Telecommunications Union (ITU) standard G692, which has a fixed point at 193.1 THz and inter-channel spacings at an integer multiple of 50 GHz or 100 GHz. An example dense WDM (DWDM) system could have a 50 GHz channel spacing and range from 191 THz to 196 THz (1525-1560 nm). One of the benefits of multi-section diode lasers is their wavelength tunability. Each section of the laser diode is supplied with a drive current, and the lasing wavelength is a function of the set of drive currents, this function generally being quite complex. Setting the output wavelength of such a laser is thus usually performed by a sophisticated microprocessor controlled control system. As well as the fact that there is a complex relation between output wavelength and the set of drive currents, there is the additional factor that wavelength switching of the laser destroys its thermal equilibrium, which results in transient wavelength instabilities until thermal equilibrium is reached at the new set of drive currents. The time needed for temperature stabilisation can be quite long.
The transient thermal properties consist of two main effects. A first effect is that, directly after the laser is switched, the thermal gradient across the device to the heat sink upon which it is mounted will be different to that measured at steady state operating conditions for these currents, due to a different heating level generated in the laser as the currents are different. This steady state temperature gradient will reassert itself over a period measured in a timescale from a few hundred nanoseconds to tens of microseconds. Because the device is at a different temperature during this period some temperature tuning of the wavelength occurs. For a positive (negative) change in tuning current the change in temperature will be such that the device is initially colder (hotter) than at equilibrium for those currents and some time will pass before the extra current dissipates enough heat energy to change this. During that period the device will be colder (hotter) than expected so a blue (red) shift from the expected output wavelength will occur.
A second effect takes place over a much longer timescale. The laser is thermally connected to a heat sink of finite thermal mass which has a temperature controller maintaining its temperature. The temperature controller cannot react instantaneously to a change in temperature, which means that with an increase (decrease) in bias current, the heat sink will heat up (down). This in turn means that for a given temperature gradient the device will have a different temperature, because the temperature the gradient is referenced from will be different. This temperature change results in the temperature of the device overshooting and going higher (lower) than would be normal for those currents. This effect may persist until the temperature controller returns the heat sink to its normal temperature, which may take 1-1.5 seconds.